Hi, a friend just gave me a pair of 12" speakers from an old Wurlitzer. They have a small square magnet on the silver frame and have 1237-? stamped on the cone.
I would be almost certain that the square manet would indicate an alnico magnet , not sure whatbrand though .. Organs are VERY HARD ON SPEAKERS because of the sustain of the note .. as opposed to the decay of a guitar note